1. Field of the Invention
The present invention relates generally to controlling the peak power demand in an electrical power distribution network by controlling the power consumption of individual loads such as air conditioning loads and, more particularly, to a system for remotely overriding the thermostats of individual space conditioning apparatus to control the temperature in the space in a predetermined manner based on external commands but without the necessity of changing or modifying the existing thermostat.
2. Description of the Prior Art
One of the most serious problems confronting electrical utility companies today is the great variance in total electrical demand on a network between peak and off-peak times during the day. Peak demand periods are intervals of very high demand on the power generating equipment where load shedding may be necessary to maintain proper service to the network. These occur, for example, during hot summer days occasioned by the wide spread simultaneous usage of electrical air conditioning devices or during the coldest winter months in areas where the usage of electrical heating equipment is prevalent.
Electric utility companies have turned to load shedding as a means of controlling peak demand and this has led to the use of the term "load-shedding interval". It is desirable that a load-shedding device reduce power demand uniformly over the entire load-shedding interval because the actual peak of power demand on the total utility grid could occur at any time during the load-shedding interval.
In the prior art it is known to send signals either over the power lines or by utilizing a radio-type signal emanating from the utility to disconnect or interrupt the use of certain selected electric loads such as airconditioning compressors when the demand has reached a certain limit.
One such system is illustrated and described in U.S. Pat. No. 4,228,511 to Simcoe et al dated Oct. 14, 1980. That application illustrates and describes the method of controlling the electrical power demand which includes assuming control of the setpoint function of the space-conditioning thermostat associated with the load at the beginning of a load-shedding interval. This may be in response to a radio signal. The overriding control functions to cause the setpoint to change at a constant fixed rate in a direction toward less power consumption to achieve load shedding over the load-shedding interval. In this manner the temperature is ramped at a preset fixed ramping rate.
Such systems provide satisfactory methods for the control of power consumption during load-shedding intervals. However, all such systems require either the replacement of the existing thermostat in the conditioned space with one specifically designed to accommodate the overriding control of the installation of a control module auxiliary to the existing thermostat in the conditioned space. These replacements or modifications may also require expensive additional wiring in existing installations. Thus, there exists a definite need for a low cost, simple system which can accomplish much of the function of the more elaborate systems without the need for expensive modification or replacement of existing temperature control equipment.